Jason is the name of a joint
CNES/NASA oceanography mission series with the objective to monitor
global ocean circulation, discover the tie between the oceans and
atmosphere, improve global climate predictions, and to monitor events
such as El Niño conditions and ocean eddies. The oceanography
mission series is considered a cornerstone of GCOS (Global Ocean
Observing System), a concept advocated by WMO (World Meteorological
Organization), IOC (Intergovernmental Oceanographic Commission of the
UNESCO), UNEP (United Nations Environmental Program), and ICSU
(International Council of Scientific Unions). 1)2)3)4)5)

Note: Jason-1 is named after the
mythological hero who led the Argonauts on the adventurous and
hazardous search for the Golden Fleece which they found and returned.
“Jason” symbolizes both the hard-fought quest for a worthy
goal and civilization's fascination with the ocean and its mysteries.

Background: In December 1996, CNES
and NASA formally agreed in a Memorandum of Understanding (MOU) to
jointly participate in the Jason mission to design, build, deploy, and
operate a satellite to continue the collection of seasurface elevation
measurements originally begun by the TOPEX/Poseidon mission. The
TOPEX/Poseidon (T/P) mission, launched in August 1992, was the first
such mission in a world-wide effort to study and describe global ocean
dynamics and its relationship to the Earth's environment and climate
change. The unprecedented success of T/P led mission planners to
recognize the need to sustain the high accuracy measurements of sea
surface elevation in order to integrate this information into climate
models for long-term climate prediction.

Jason-1 was successfully launched in
December 2001, and since then is delivering geophysical data at a level
of performance identical to the T/P mission (note: the T/P mission
ceased operations in the fall 2005 after 13 years of operations). Jason-1 is noticeably different from T/P in that the measurements are no longer experimental in nature;near-real-time products derived from the altimetric measurements are being disseminated on a routine basis. 6)

Jason-1 spacecraft

The Jason-1 minisatellite carries a
radar altimeter, a follow-on mission to the TOPEX/Poseidon mission.
Jason-1 is the first spacecraft to use the Proteus minisatellite bus, a
multimission platform of CNES/Alcatel Space Industries (partnership).
The overall architecture uses redundancy by sharing two half
satellites, each under the control of one data handling unit. The
satellite consists of the payload module (which accommodates the
various instruments) and the Proteus platform, a boxlike structure of 1
m side lengths, which provides on-board services. The hydrazine
propellant system uses a 28 l tank with 4 x 1 N thrusters (propulsion
is only used for orbit acquisition and maintenance).

Redundancy concept: The functional
redundancies are fully ensured at the satellite level. As far as the
hardware is concerned, the equipment units are either one-to-one or
“n” out-of “m” redundant (for example: 2 gyros
out of 3, 3 reaction wheels out of 4...). However, the most important
point to mention is the one to one Processor Module / Data Handling
Unit (DHU) redundancy concept referred to satellite’s halves.
This concept is illustrated in the Table 1 below showing the shared
elements versus the ones separated between the two halves. For
instance, it is important to understand that a loss of transmitter
“Tx1” will yield to the complete loss of the
“A” side even though the Processor Module is still
“alive”.

The Jason-1 operational orbit
follows an exact repeat ground track every 127 revolutions in 10 days
with the same characteristics than that of Topex/Poseidon (identical
orbital tracks (about a minute apart) to perform cross calibration). In
this tandem mission setup, Jason-1 is located one minute ahead of Topex/Poseidon. 9)

Jason-1, although a minisatellite,
is a true ocean observatory providing SSH (Sea Surface Height) and
sea-state measurements in near-real time (within 3 hours) to an
international user community. The major science objectives of Jason-1
mission applications are:

• Oceanography and ocean
forecasting. An operational application example is the prediction of
ocean currents including eddies in ocean circulation spanning distances
up to 100 km.

• Geophysics. The Earth's
gravity field affects sea level. By measuring the ocean's dynamic
topography, we can learn more about plate tectonics, bottom topography,
movements of the Earth's mantle and many other geophysical phenomena.
Altimetric data are also used to study ice, lakes and rivers, and
relief in desert zones.

Table 2: Comparison of mass/power budgets for the Topex/Poseidon and Jason-1 S/C 10)

Figure 5: Overview of altimetry missions, program status as of Sept. 2012 11)

Jason-1 mission status:

• Dec. 29, 2015: It has been
said that we have more complete maps of the surface of Mars or the Moon
than we do of Earth. Close to 70 percent of our planet is covered by
water, and that water refracts, absorbs, and reflects light so well
that it can only penetrate a few tens to hundreds of meters. To humans
and most satellite eyes, the deep ocean is opaque. 12)13)

- But there are ways to visualize
what the planet looks like beneath that watery shroud. Sonar-based
(sounding) instruments mounted on ships can distinguish the shape
(bathymetry) of the seafloor. But such maps can only be made for places
where ships and sonar pass frequently. The majority of such
measurements have been made along the major shipping routes of the
world, interspersed with results from scientific expeditions over the
past two centuries. About 5 to 15 percent of the global ocean floor has
been mapped in this way, depending on how you define
“mapped.”

- There is another way to see the
depths of the ocean: by measuring the shape and gravity field of Earth,
a discipline known as geodesy. David Sandwell of SIO (Scripps
Institution of Oceanography) and Walter Smith of NOAA (National Oceanic
and Atmospheric Administration) have spent much of the past 25 years
negotiating with military agencies and satellite operators to allow
them acquire or gain access to measurements of the Earth’s
gravity field and sea surface heights. The result of their
collaborative efforts is a global data set that tells where the ridges
and valleys are by showing where the planet’s gravity field
varies.

Legend to Figure 6:
Shades of orange and red represent areas where seafloor gravity is
stronger (in milligal) than the global average, a phenomenon that
mostly coincides with the location of underwater ridges, seamounts, and
the edges of Earth’s tectonic plates. Shades of blue represent
areas of lower gravity, corresponding largely with the deepest troughs
in the ocean.

Figure 7: This map shows a tighter view of that data along the Mid-Atlantic Ridge between Africa and South America(image credit: NASA Earth Observatory, Joshua Stevens, Ref. 12)

- The maps were created through
computer analysis and modeling of new satellite altimetry data from the
European Space Agency’s CryoSat-2 and from the NASA/CNES Jason-1,
as well as older data from missions flown in the 1980s and 90s.
CryoSat-2 was designed to collect data over Earth’s polar
regions, but it also collected measurements over the oceans. Jason-1
was specifically designed to measure the height of the oceans, but it
had to be adjusted to a slightly different orbit in order to acquire
the data needed to see gravity anomalies.

- But how does
the height of the sea surface (which is what the altimeters measured)
tell us something about gravity and the seafloor? Mountains and other
seafloor features have a lot of mass, so they exert a gravitational
pull on the water above and around them; essentially, seamounts pull
more water toward their center of mass. This causes water to pile up in
small but measurable bumps on the sea surface. (If you are wondering
why a greater mass would not pull the water down, it is because water
is incompressible; that is, it will not shrink into a smaller volume.)

- The new measurements of these tiny
bumps on the sea surface were compared and combined with previous
gravity measurements to make a map that is two- to four times more
detailed than before. Through their work, Sandwell, Smith, and the team
have charted thousands of previously uncharted mountains and abyssal
hills. The new map gives an accurate picture of seafloor topography at
a scale of 5 km/pixel.

- From these seafloor maps,
scientists can further refine their understanding of the evolution and
motion of Earth’s tectonic plates and the continents they carry.
They can also improve estimates of the depth of the seafloor in various
regions and target new sonar surveys to further refine the details,
especially in areas where there is thick sediment. The map of Figure 8 shows the gravity data as a cartographer would represent the seafloor, with darker blues representing deeper areas.

Figure 8:
The new map, acquired in 2014 from data of several altimetry missions,
gives an accurate picture of seafloor topography at a scale of 5
km/pixel ((image credit: NASA Earth Observatory, Joshua Stevens, Ref. 12)

• Spring 2014: Despite the
robustness of the platform -initially designed for 3 years- Jason-1
underwent several hardware setbacks over the course of its extended
lifetime, most of them due to the expected harsh radiative environment.
Some of them could have turned to be life-threatening for its mission,
but each time an operational work-around solution has been found to
push the envelope a little further. This was made possible thanks to
the deep knowledge of the platform by TAS (Thales Alenia Space) and
CNES experts, as well as to the awareness, preparedness, reactivity and
creativity of the operational teams. 14)15)

Each time, the operational teams had
to think out of the box to find a way to mitigate the issue, either by
conveniently using available redundancies, developing software patches,
or adapting operational strategies (or a combination of those, most of
the time), without compromising on the safety of decommissioning
operations.

Another factor to take into account
was the sharing of operations between JPL (routine and first step of
Safe Hold Mode recovery) and CNES (contingency operations and steps 2/3
of Safe Hold Mode recovery): for these joint operations to work
smoothly, it was crucial to put in place simple and robust procedures
for emergency operations to be performed by JPL.

Thanks to this fruitful cooperation,
these contingencies were almost transparent to the mission, and Jason-1
was able to deliver great science and fully satisfy the scientific
community for over 11 years. On top of it, many lessons learned have
been taken for the remainder of the ageing PROTEUS fleet (especially
for Jason-2 and the upcoming Jason-3): some solutions elaborated on
Jason-1 will be directly applicable, or will have to be slightly
adapted based on the particular on-board context, and it will be
possible to anticipate and act as early as possible when a problem
occurs.

• End of the Jason-1 mission in late June 2013 after 11 ½ years of operations:
The successful joint NASA/CNES Jason-1 ocean altimetry satellite was
decommissioned on July 1, 2013 following the loss of its last remaining
transmitter. From 2001 to 2013, Jason-1 provided a major contribution
to the monitoring of sea level rise, an essential climate variable.
This was due to its excellent measurement accuracy, the long-term
stability of its instruments, and the continuous effort of
calibration-validation performed on the ground. 16)17)

- Launched on Dec. 7, 2001, and
designed to last three to five years, Jason-1 helped create a
revolutionary 20-plus-year climate data record of global ocean surface
topography that began in 1992 with the launch of the NASA/CNES
TOPEX/Poseidon satellite. For more than 53,500 orbits of our planet,
Jason-1 precisely mapped sea level, wind speed and wave height for more
than 95 percent of Earth's ice-free ocean every 10 days. The mission
provided new insights into ocean circulation, tracked our rising seas
and enabled more accurate weather, ocean and climate forecasts.

- Contact was lost with the Jason-1
satellite on June 21, 2013 when it was out of visibility of ground
stations. At the time of the last contact, Jason-1 and its instruments
were healthy with no indications of any alarms or anomalies. Subsequent
attempts to re-establish spacecraft communications from U.S. and French
ground stations were unsuccessful. Extensive engineering operations
undertaken to recover downlink communications also were unsuccessful.

- After consultation with the
spacecraft and transmitter manufacturers, it was determined a
non-recoverable failure with the last remaining transmitter on Jason-1
was the cause of the loss of contact. The spacecraft's other
transmitter experienced a permanent failure in September 2005. There
now is no remaining capability to retrieve data from the Jason-1
spacecraft.

- On July 1, 2013, mission
controllers commanded Jason-1 into a safe hold state that reinitialized
the satellite. After making several more unsuccessful attempts to
locate a signal, mission managers at CNES and NASA decided to proceed
with decommissioning Jason-1. The satellite was then commanded to turn
off its magnetometer and reaction wheels. Without these attitude
control systems, Jason-1 and its solar panels will slowly drift away
from pointing at the sun and its batteries will discharge, leaving it
totally inert within the next 90 days. The spacecraft will not reenter
Earth's atmosphere for at least 1,000 years.

During parts of its mission, Jason-1
flew in carefully coordinated orbits with both its predecessor,
TOPEX/Poseidon, and its successor, Jason-2/OSTM ( Ocean Surface
Topography Mission), launched in 2008. These coordinated orbit periods,
which lasted about three years each, cross-calibrated the satellites,
making possible a 20-plus-year unbroken climate record of sea level
change. These coordination periods also doubled data coverage.

Combined with
data from the ESA's Envisat mission, which also measured sea level from
space, these data allow scientists to study smaller-scale ocean
circulation phenomena, such as coastal tides, ocean eddies, currents
and fronts. These small-scale features are thought to be responsible
for transporting and mixing heat and other properties, such as
nutrients and dissolved carbon dioxide, within the ocean.

The in orbit Jason-2 mission,
operated by the meteorological agencies NOAA and EUMETSAT in
collaboration with NASA and CNES, is in good health and continues to
collect science and operational data. This same U.S./European team is
preparing to launch the next satellite in the series, Jason-3, in March
2015 (Ref. 16).

•
Jason-1 completed 50,000 orbits on September 18, 2012. Jason-1
continues to make an essential contribution to ocean surface topography
and to geodesy. 19)

•
Jason-1 has been moved to a lower orbit and began its geodetic mission
on 7 May 2012. The core payloads were switched ON on May 4th and after
some POSEIDON-2 radar (PRF) adjustments, the mission was resumed on May
7th at 15:12:48 UTC. 20) In this new operational phase the Jason-1 mission is in a drifting geodetic orbit.21)

- No longer on the interleaved track

- Revisit time > 400 days (end of the 10-day exact repeat cycle).

Semi major axis

7702.437 km

Eccentricity

1.3 to 2.8 x 10-4

Altitude at equator

1324.0 km

Orbital period

6630 s (or 1 h 52' 10'')

Inclination

66.042º

Cycle

406 days

Sub-cycles

3.9, 10.9, 47.5, 179.5 days

Table 3: Characteristics of the new orbit of Jason-1

•
April 23, 2012. The Jason-1 project teams at CNES and NASA/JPL have now
completed all prerequisite tasks required to enable a safe recovery of
the mission from the safehold it has been in since March 3, 2012. 22)

•
At the end of February and in early March 2012, Jason-1 encountered an
anomaly putting the spacecraft into safehold mode. CNES and NASA
management, through the Joint Steering Group, have directed the Jason-1
Project to begin a series of maneuvers to reduce the orbit on a
drifting geodetic orbit at 1324.0 km. This orbit is a drifting orbit
with a cycle of 406 days and sub-cycles of 3.9 - 10.9 - 47.5 - 179.5
days. 23)

• The Jason-1 mission is
operating nominally in 2012. Jason-1 continues to acquire high quality
data and the interleaved data of Jason-1 and Jason-2/OSTM are
supporting important operational applications and new scientific
investigations of mesoscale variability. Additional science
contributions will occur when Jason-1 moves to a geodetic orbit (1336
km) to provide estimates of the marine geoid and ocean bottom
topography (this may occur after the AltiKa data of the SARAL
missionhave been verified by Jason-2/OSTM, sometime in 2012).
In June 2011, the NASA Earth Science Senior Review recommended an
extension of the Jason-1 mission as baseline to 2013 and for
augmentation to 2015. 24)

•
On Dec. 7, 2011, the Jason-1 satellite celebrated 10 years on-orbit,
adding to a 20-year continuous satellite record of global sea level
rise and monitoring the waxings and wanings of El Nino and La Nina. 25)26)

• Jason-1 is operating nominally in 2011 (extended mission in its 10th year on-orbit). The Jason-1 retirement depends on the SARAL/AltiKa post-launch readyness date. 27)
Note: The SARAL/AltiKa mission (ISRO and CNES) is scheduled for launch on PSLV-C20 in the spring of 2012.

- Loss of gyro No 1 in March 2010 (Ref. 27) And switch to gyro No 3 in April 2010.

• Jason-1 is operating nominally in 2010 (well beyond its design life). In 2009, a mission extension was given to 2011. 28)

• In late July 2010, an
operation of tank emptying was started. For data quality reasons and
due to the importance of that mission, the project wants to operate
Jason-1 on this operational orbit in conjunction with Jason-2 up to
arrival of new oceanography satellites such as SARAL. 29)

• Jason-1
is operating nominally in 2009 (completion of 7th year in orbit on Dec.
7, 2008). However, after providing services on the nominal ground track
for 7 years, the Jason-1 satellite was moved to a new interleaved orbit
with Jason-2 at the end of repeat cycle 259 (Jan. 26, 2009) - the
TOPEX/Poseidon mission used this same ground track in the period
2002-2005.30)31)

The Jason-2 satellite will continue
the long-term climate data record on the primary TOPEX/Poseidon /
Jason-1 / Jason-2 ground track. The interleaving of the Jason-1 with
the Jason-2 orbits will provide significant advantages for operational
applications.
Repeat cycle 262 (mid-February 2009) was the first Jason-1 cycle on the
new interleaved ground track. The new orbit is phased 162º ahead
of Jason-2 with a time lag of approximately five days. The first
complete science cycle in the new Jason-1 orbit (Cycle 263) started at
04:18 UTC on Feb 20, 2009. Nominal production of all data products in
the new orbit resumed on Feb. 14, 2009. 32)33)34)

• Current estimates (Nov. 2008)
of CNES/Thales are that the Jason-1 spacecraft will provided its
services beyond April 2010 with a probability of 77% (Ref. 48).

• Safe hold an recovery: On
Aug. 7, 2008, Jason-1 went to a safe hold after experiencing a SEU on a
relay leading to reaction wheel #3 (RW3). Recovery was completed on
Aug. 13, 2008 and nominal operations were resumed (Ref. 48).

•
The Jason-1 spacecraft was kept on the same ground track as
TOPEX/Poseidon (launch Aug. 10, 1992) until August 15, 2002. Then,
through some orbit maneuvers, TOPEX/Poseidon was slowly shifted to be
midway between its former tracks (with only 1 minute separation between
the two spacecraft), which it reached on September 20, 2002. This
orbital configuration provided interleaved ground tracks with a
1.4º longitude spacing from the Jason-1 tracks - permitting
frequent cross-calibrations which resulted in ocean topography data
with unprecedented accuracy (better than either S/C could attain by
itself). The inter-satellite calibration has determined the relative
bias to an accuracy of 1.6 mm. Results from the merging of T/P and
Jason-1 data have confirmed the potential of an optimized two-satellite
configuration for mesoscale variability studies. - The tandem mission
of Jason-1 and TOPEX/Poseidon lasted until Oct. 2005, when the
TOPEX/Poseidon mission ended due to a failure in a pitch reaction
wheel. 35)36)37)38)39)

• In Dec.
2004, Jason-1 achieved its primary mission goal of 3 years of
operations (mission design life). This is now being followed by the
extended Jason-1 mission. The extended mission of Jason-1 is expected
to last to overlap with the Jason-2 mission with an expected launch in
mid-2008 (to provide a cross-calibration opportunity).

Figure 9: Artist's view of the TOPEX/Poseidon and Jason-1 tandem mission and sea level rise (image credit: University of Colorado)

Legend to Figure 9:
The global sea level has risen about 3 mm per year since Topex/Poseidon
(on the left) began its precise measurement of sea surface height in
1993 and was followed by Jason-1 in 2001. In this Figure, the vertical
scale represents the globally averaged sea level (mm). Seasonal
variations in sea level have been removed to show the underlying trend.

• On November 19, 2003 at
03:58, Jason-1 transitioned to Safe Hold Mode following the triggering
of the onboard “Reaction Wheel Continuity monitoring” FDIR
(1Hz monitoring, filter set to 3). — A group of experts was
formed to gather all relevant observations and previous analyses
performed in-flight or during the development phase, to research the
most probable causes, to assess impacts on on-board systems robustness,
and proposed recommendations (operational ones on Jason-1, as well as
for the development of Proteus satellites at that time, Ref. 14).

Sensor complement: (Poseidon-2, JMR, TRSR, LRA)

The payload module provides mechanical, electrical, thermal and dynamical support to the Jason instruments.

Poseidon-2 (Solid-State Radar Altimeter):

Poseidon-2 is a dual-frequency
nadir-looking radar altimeter with the objective to map the topography
of the sea surface for calculating ocean surface current velocity and
to measure ocean wave height and wind speed. Poseidon-2 is a
CNES-sponsored instrument designed and developed by Alcatel Alenia
Space (formerly Alcatel Espace) as prime contractor. Poseidon-2 is of
SSALT/Poseidon-1 heritage on TOPEX/Poseidon.

The Poseidon-1 instrument is being
upgraded for the Jason radar altimeter mission by adding a second
C-band frequency of 5.3 GHz (Ku-band at 13.575 GHz), as well as
changing to digital technology and using a new radiation-hardened
microprocessor (the altimeter electronics are split into two boxes, the
PCU (Processing & Control Unit), and the RFU (Radio Frequency
Unit). The PCU includes a chirp generator, baseband demodulator,
spectrum analyzer, instrument control unit and interfaces. The RFU
performs the up-conversion to Ku- and C-bands, the high power
solid-state amplification, the low-noise amplification of the received
echoes, and its mixing with a reference chirp. The C-band channel
provides direct ionospheric correction for the primary Ku-band
measurement and uses a 1.2 m antenna on the nadir side of the
spacecraft. 40)41)42)43)

The major evolutions of the Poseidon-2 altimeter with respect to SSALT/Poseidon-1 are:

• Addition of the second frequency (C Band) to obtain direct ionospheric correction.

• The digital chirp generator to allow a selectable bandwidth necessary for the C band channel and a better phase knowledge

• Increase of the emitting
power to keep an acceptable link budget (the size of the antenna has
been decreased with respect to Topex/Poseidon)

• A 128 points FFT transform to obtain a better mispointing evaluation and then reduce the pointing requirements

• Use of a rad hard
microprocessor to provide a better radiation tolerance and thus
increase the operational availability of the sensor. This
microprocessor is also more powerful to increase the on-board
processing capability of the altimeter.

JMR is a JPL instrument of TMR
heritage. JMR is a passive microwave radiometer measuring the
brightness temperatures in the nadir column at 18.7, 23.8, and 34 GHz,
providing path delay correction for the altimeter (the brightness
temperatures are converted to path-delay information). The 23.8 GHz
channel is the primary water vapor sensor, the 34 GHz channel provides
a correction for non-raining clouds, and the 18.7 GHz channel provides
the correction for effects of wind-induced enhancements in the sea
surface background emission. 44)45)46)

DORIS is a CNES/Thomson development.
DORIS is a precision orbit determination system providing position and
ionospheric correction for Poseidon-2. Doris measurements are also used
for geophysical studies, in particular through the International Doris
Service (IDS). Doris is a dual-frequency instrument able to determine
atmospheric electron content.

The DORIS flight segment consists of
a two-channel, two-frequency (401.25 MHz and 2036.25 MHz) Doppler
receiver capable of tracking signals from a worldwide network of about
50 ground beacons. The Jason DORIS receiver is the same second
generation device as the one developed for the ENVISAT mission. Its
main functional improvements over first-generation receivers are its
capability to receive two beacons simultaneously and to produce onboard
the orbit ephemeris in real time with a precision of 1 m. The receiver
is controlled by an ultra-stable oscillator delivering the reference
frequency with a stability of 5 x 10-13 over a 10-100 second
interval and delivering an on-board time output within 0.1 ms accuracy.
The DORIS instrument mass is 31 kg, power = 30 W.

Figure 13: Illustration of the DORIS antenna (image credit: AVISO)

BlackJack (GPS Flight Receiver):

BlackJack is also referred to as TRSR-2
(Turbo Rogue Space Receiver-2). The instrument is of GPS/MET (Microlab)
heritage (of a design as flown on CHAMP) and is being provided by
NASA/JPL and built by Spectrum Astro Inc. of Gilbert, AZ. BlackJack is
a 16-channel GPS receiver with the objective to provide supplementary
positioning data to DORIS in support of the POD (Precision Orbit
Determination) function and to enhance and/or improve gravity field
models. Radial accuracies of 1-2 cm are obtained in post-processing.
BlackJack is a fully redundant unit (two independent receivers
operating in cold redundancy). Each unit is comprised of an
omnidirectional antenna, low-noise amplifier, crystal oscillator,
sampling down-converter, and a baseband digital processor assembly,
communicating through a 1553 bus interface. Instrument mass = 10 kg
(2), power = 17.5 W.

In its current configuration, the
BlackJack on Jason-1 can track up to 12 GPS satellites simultaneously
in dual-frequency mode. From these signals, BlackJack acquires
measurements of the GPS carrier phase providing range measurements with
an accuracy of about 1 mm; the absolute pseudo range (defined as the
absolute range plus receiver time offset from GPS time) has an accuracy
of about 10 cm. BlackJack provides also onboard solutions for S/C
position and time, accurate to about 50 m and 150 ns, respectively. 47)

Note. As fall of 2008, the life expectancies of the two TRSR receivers has been surpassed. 48)

a JPL instrument of TOPEX/Poseidon
heritage, built by ITE Inc. under NASA/GSFC contract. LRA provides a
reference target for satellite laser ranging (SLR) measurements, which
are necessary to calibrate the POD system and the altimeter throughout
the mission. The LRA is placed on the nadir face of the satellite. It
is a totally passive unit that consists of nine quartz corner cubes
arrayed as a truncated cone with one in the center and the other eight
distributed azimuthally around the cone. This arrangement allows laser
ranging at FOV (Field-of-View) angles of 360º in azimuth and
60º elevation around the perpendicular. The retroreflectors are
optimized for a wavelength of 532 nm (green), offering a FOV of about
100º. The LRA instrument mass is 2.2 kg.

Figure 15: Illustration of LRA (image credit: NASA)

The LRA is a passive instrument that
acts as a reference target for laser tracking measurements performed by
ground stations. Laser tracking data are analyzed to calculate the
satellite's altitude to within a few millimeters. However, the small
number of ground stations and the sensitivity of laser beams to weather
conditions make it impossible to track the satellite continuously. That
is why other onboard location systems are needed.

Ground segment:

The Jason-1 ground segment is made
up of three components providing the human (teams) and equipment
resources required to ensure the mission's success: 49)50)

The Jason-1 spacecraft has been
monitored and controlled from CNES in Toulouse up to the end of the
assessment phase (first 30/50 days of the mission). - Then the POCC
(Project Operations Control Center) of JPL took over.

The baseline ground network includes
stations at Aussaguel (France), Fairbanks (AK, Poker Flats location),
Wallops Island, VA, and Hartebeesthoek, South Africa. In addition, the
Altimetric and Orbitography Mission Center (SSALTO), located in
Toulouse and the Jason Science Data System at JPL in Pasadena, form the
mission ground segment.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).